Thread and shared lock

1.主线程执行，开启5个子线程，进行计数，没有使用mutex锁住，由于没有lock住，每个线程对全局变量的操作错乱，结果如下：

 1 """
 2 synchronize access to stdout: because it is shared global
 3 thread outputs may be intermixed if not syschronized
 4 """
 5 import thread,time
 6 global num                                  #global var to be used by many threads
 7 num=0
 8
 9 def cnt(id,count):                          # function run in threads
10     for i in range(count):
11         global num
12         #mutex.acquire()                     # lock the share var before execute
13         num +=1
14         time.sleep(0.5)                     # simulate read work
15         print(‘[%s] num= %s\n‘ %(id,num))   #print isn‘t interrupted now
16         #mutex.release()                     #release the lock for the other thread
17
18 if __name__ =="__main__":
19     #mutex=thread.allocate_lock()            #make a global mutex for lock
20     for i in range(5):                      #spawm 5 threads
21         thread.start_new_thread(cnt,(i,3))  #start threads
22     time.sleep(8)                          # wait for spawn thread work done,don‘t exit too early
23
24     print(‘main thread exitting‘)

2.把mutex 注释打开，有了mutex变量，每一个线程进入都会独占num变量，结果如下：

 1 """
 2 synchronize access to stdout: because it is shared global
 3 thread outputs may be intermixed if not syschronized
 4 """
 5 import thread,time
 6 global num                                  #global var to be used by many threads
 7 num=0
 8
 9 def cnt(id,count):                          # function run in threads
10     for i in range(count):
11         global num
12         mutex.acquire()                     # lock the share var before execute
13         num +=1
14         time.sleep(0.5)                     # simulate read work
15         print(‘[%s] num= %s\n‘ %(id,num))   #print isn‘t interrupted now
16         mutex.release()                     #release the lock for the other thread
17
18 if __name__ =="__main__":
19     mutex=thread.allocate_lock()            #make a global mutex for lock
20     for i in range(5):                      #spawm 5 threads
21         thread.start_new_thread(cnt,(i,3))  #start threads
22     time.sleep(8)                          # wait for spawn thread work done,don‘t exit too early
23
24     print(‘main thread exitting‘)

3.如果把time.sleep(6)注释掉或者子线程没有执行完毕，而主线程sleep的时间一到，主线程直接退出而不等待子线程执行完毕，结果如下：

a.主线程不等待，则直接退出

b.主线程只等待3s，而5个子线程需要7.5s，所以num只计数5.

4.设定有效等待时间和锁之后，主线程等待所有子线程执行结束才退出，结果如下：